There are usually 2-3 bulbs behind the unit, depending on the exact model. My 2001 had 3 for the manual HVAC unit. I'm not sure if it is 2 or 3 for the automatic.
They are fairly easy to replace, if you are willing to be somewhat patient. There's a couple threads on this forum (can google "site:subaruoutback.org HVAC bulbs") that go over how to replace them in a 1997/1998 model which should be exactly what you want.